The invention relates generally to a method for the electroless nickel plating of aluminum and its alloys.
Electroless nickel plating is a process which is very important in the metal finishing industry and which is widely employed for many metal substrates, including steel, copper, nickel, aluminum and alloys thereof. Plating metals such as aluminum, magnesium and their alloys present special problems to electroplaters, however, because, for one, they have surface oxide coatings which require special pre-plating operations to condition the surface for plating. While the present invention is applicable to the electroless plating of such metal substrates with metals such as nickel, cobalt and nickel-cobalt alloys, the description which follows will be primarily directed for convenience to the electroless nickel plating of aluminum and aluminum alloys which have been conditioned for plating by depositing a zinc coating on its surface.
In general, aluminum parts are first cleaned to remove organic surface contamination, followed by etching to eliminate solid impurities and alloying constituents from the surface, desmutting to remove the oxide film, and coating with a barrier layer such as zinc or tin to prevent re-oxidation of the cleaned surface. The parts are usually rinsed after each of the above steps and are now ready for electroless nickel plating.
Unfortunately, however, the elctroless nickel plating bath used to plate zincated aluminum has a relatively short bath life when compared to baths used to plate many other metal alloys such as plain steel. Thus, a bath which would normally be useful for, as an example, about ten turnovers for steel, may be useful on barrier coated aluminum for only about five turnovers. After this it must be discarded and replaced because the nickel deposits on the aluminum start to be blistered. A turnover may be defined as the period during which the quantity of nickel metal that has been plated out is equal to the quantity of nickel in the bath as made up. For example, for a bath initially containing about 6 g/l nickel, the bath would usually be replenished with nickel salts back to 6 g/l as the nickel is consumed during plating. The cumulative replenishment of 6 g/l nickel represents one turnover.
Zincating is a commercially important process to pretreat aluminum surfaces because it is a relatively simple process requiring only immersion of the aluminum part in alkaline solution containing zincate ions. The amount of zinc deposited is actually very small and depends on the time and type of immersion bath used, the aluminum alloy, temperature of the solution and the pretreatment process; thicknesses up to about 0.1 microns are usually employed.
An alternative to the zincate process is shown in U.S. Pat. No. 3,666,529 to Wright et al. which discloses a method of conditioning aluminum surfaces bascially comprising etching the aluminum with an acidic nickel chloride solution to expose the aluminum crystals and deposit a nickel coating, removing the nickel coating with HNO.sub.3, activating with an alkaline solution containing hypophosphite ions and then electrolessly plating an alkaline strike coat of nickel at 85.degree. to 90.degree. C., followed by electroless nickel deposition to the desired nickel thickness.
U.S. Pat. No. 3,672,964 to Bellis et al. discloses pretreating the aluminum surfaces with an aqueous solution of hydrofluoric acid and a material which is displaced by the aluminum and which is active to the electroless plating nickel, thereafter plating the treated aluminum surface with an electroless nickel bath which is at a pH of 6-7 and contains an amine borane and a monovalent or divalent sulfur compound. These patents however, do not address themselves to the problems encountered in the electroless nickel plating of zincated aluminum and only provide alternative processes which may be more costly and time consuming.